docs: def com_adobe_fonts_check_family_consistent_upm(ttFonts):
      """Fonts have consistent Units Per Em?"""
      upm_set = set()
      for ttFont in ttFonts:
          upm_set.add(ttFont['head'].unitsPerEm)
      if len(upm_set) > 1:
          yield FAIL, ("Fonts have different units per em: {}."
                       ).format(sorted(upm_set))
      else:
          yield PASS, "Fonts have consistent units per em."